Straw is the above ground part of cereal and grass seed plants remaining after the grain or grass seed has been removed. Species of cereals and grasses providing straw include wheat, rice, rye, oats, barley, fescue, annual and perennial ryegrass, bluegrass and bentgrass. Although straw has been available in substantial quantities since the dawn of agriculture, only recently have straw-based cellulosic materials been considered suitable for use in modem building materials. In many areas, the use of straw as a building material was not permitted due to a common perception that straw is an inherently inferior building material. Unlike wood-based cellulosic materials, which have been used successfully for centuries, straw has not generally been considered useful because of the perception that straw lacked strength, durability, or fire retardance.
Contrary to conventional wisdom, however, recent experience with straw as a building material has shown that, employed correctly, straw can be used very effectively in modern construction. One current method of the use of straw in construction involves the incorporation of entire straw bales into the walls of a house. Experience with this method has shown that sufficiently dense packing and size can provide the necessary strength and structural support required for home construction.
Another current straw-based building material is a thin panel of compressed straw combined with a resin binder. Such panels have been shown to be useful as a core layer or core stock in a plywood laminate. These panels are then incorporated with stronger wood laminate layers for the production of plywood. The cellulose fibers used in the production of the plywood may be derived from pulp, waste paper, spoiled paper, pulp sludge, linter, bagasse, and other such materials in addition to those derived from straw.
There remains the problem, however, with the inherent flammability of cellulose-based materials. Traditional cellulose-based flame retardant materials have inherent drawbacks owing to the fire-retardant additives incorporated therein. For example, an addition of 15 to 25% borate or boric acid can be added to the cellulosic stock to make the material flame retardant. The inclusion of these fire-retardant additives renders the material highly hydroscopic, or water-absorptive. In addition, these materials tend to absorb more moisture over the course of time, which can cause significant dimensional changes in structures built with such materials.
Other additives are known to improve the fire retardance of cellulose-based materials, including condensed ammonium phosphate. During processing, these chemicals are generally added to the composition before thermal curing, causing the chemical to react with or adhere to the surface of the molecules and fibers of cellulose. Although some of these materials have reduced water absorption characteristics as compared to materials using borate, concerns over the additive and waste material costs, including disposal costs, make the use of large quantities of such additives undesirable.
Furthermore, these chemical additives have not demonstrated a sufficient improvement in the fire retardance of cellulose-based materials to justify the use of these fire-retardant materials in high-risk and industrial environments. For example, these fire-retardant materials have not had sufficient fire resistance to qualify for ratings at the higher end of the classifications, such as the “45 minute” rating for doors.
Accordingly, there is a need for an inexpensive, relatively lightweight fire-retardant building material capable of qualifying for the higher end fire resistance classifications which is not excessively water absorptive, and which does not require large quantities of chemical additives for its manufacture.